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Level 2 Chemistry, 201891164 Demonstrate understanding of bonding,

structure, properties and energy changes

9.30 a.m. Monday 26 November 2018 Credits: Five

Achievement Achievement with Merit Achievement with ExcellenceDemonstrate understanding of bonding, structure, properties and energy changes.

Demonstrate in-depth understanding of bonding, structure, properties and energy changes.

Demonstrate comprehensive understanding of bonding, structure, properties and energy changes.

Check that the National Student Number (NSN) on your admission slip is the same as the number at the top of this page.

You should attempt ALL the questions in this booklet.

A periodic table is provided in the Resource Booklet L2–CHEMR.

If you need more room for any answer, use the extra space provided at the back of this booklet and clearly number the question.

Check that this booklet has pages 2 –12 in the correct order and that none of these pages is blank.

YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE EXAMINATION.

QUESTION ONE

(a) The equation for the dissolving of ammonium chloride, NH4Cl, in water is shown below.

NH4Cl(s) → NH4+(aq) + Cl–(aq) ΔrHº = +17.8 kJ mol–1

Circle the term that best describes this reaction:

endothermic exothermic

Give a reason for your choice.

(b) (i) Respiration is the process by which energy is released from glucose.

C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(ℓ)

Circle the term that best describes this reaction:

endothermic exothermic

Give a reason for your choice.

(ii) Water formed in the respiration reaction evaporates.

H2O(ℓ) → H2O(g)

Explain whether this process is endothermic or exothermic

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(c) (i) Butane is used to fuel a camping stove. Butane burns readily in oxygen.

The following is an energy profile diagram for the combustion of butane.

Reactants

Products

Energy

Reaction

ActivationEnergy

Explain how the diagram shows that the enthalpy change for this reaction is negative.

(ii) The following is the equation for the combustion of butane gas in oxygen.

2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g) ΔrHº = –5760 kJ mol–1

The fuel cylinder for the stove contains 450 g of butane gas.

Calculate the energy released when this mass of butane gas is burned completely in oxygen.

Show your working and include appropriate units in your answer.

M(C4H10) = 58.0 g mol–1

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(d) Nitrogen gas, N2(g), reacts with hydrogen gas, H2(g), to produce ammonia gas, NH3(g), as shown by the following equation:

N2(g) + 3H2(g) → 2NH3(g) ΔrHº = –92.0 kJ mol–1

Calculate the average bond enthalpy of the N–H bond in NH3, using the average bond enthalpies in the table below.

Bond Average bond enthalpy kJ mol–1

N≡N 945H–H 436

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QUESTION TWO

(a) Draw the Lewis structure (electron dot diagram) for each of the following molecules, and name their shapes.

Molecule H2S NH3 BF3

Lewis Structure

Name of Shape

Approximate bond angle around central atom

109.5° 109.5° 120°

(b) Compare and contrast the shapes and bond angles of NH3 and BF3.

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(c) The Lewis structures for two molecules are shown below.

Molecule H C N O C O

Polarity of molecule Polar Nonpolar

Hydrogen cyanide, HCN, is polar, and carbon dioxide, CO2, is nonpolar.

Both molecules are linear.

Explain why the polarities of the molecules are different, even though their shapes are the same.

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(d) Methanol, CH3OH(ℓ), is made industrially by reacting carbon monoxide, CO(g), and hydrogen, H2(g).

CO(g) + 2H2(g) → CH3OH(ℓ) ΔrHº = –91.0 kJ mol–1

Calculate the volume of methanol made when 4428 kJ of energy is released.

The mass of 1.00 L of methanol is 0.790 kg.

M(CH3OH) = 32.0 g mol–1

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QUESTION THREE

(a) Complete the table below by choosing the appropriate type of solid that matches the properties shown in the table.

Types of solid: Ionic, Metallic, Covalent Network, Molecular.

Solid Melting point (oC)

Boiling point (oC)

Conducts electricity? Soluble in water? Type of solid

A 290 732 solid – nomolten – yes

Yes, solution conducts electricity

B 44 280 No No

C 1710 2230 No No

D 660 2470 Solid and molten – yes No

(b) Explain why Solid A does not conduct electricity in the solid state, but will conduct when molten or when dissolved in water.

Refer to the particles, structure, and bonding of this substance.

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(c) Elaborate on the differences in the melting points of solids B and D with reference to their particles, structure, and bonding.

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Question Three continues on the following page.

(d) (i) Use an annotated diagram to show how solid A is able to dissolve in water.

Show the solid before dissolving, and the dissolving process of the solid.

(ii) Explain the attractions that allow solid A to be soluble in water.

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QUESTION NUMBER

Extra paper if required.Write the question number(s) if applicable.

911

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QUESTION NUMBER

Extra paper if required.Write the question number(s) if applicable.